The semaphorin gene family includes a large number of molecules that encode related transmembrane and secreted glycoproteins known to be neurologic regulators. The semaphorins are generally well conserved in their extracellular domains which are typically about 500 amino acids in length. Semaphorin family proteins have been observed in neuronal and nonneuronal tissue and have been studied largely for their role in neuronal growth cone guidance. For example, the secreted semaphorins known as collapsin-1 and Drosophila semaphorin II are selectively involved in repulsive growth cone guidance during development. Flies having semaphorin II genes that are mutated so that their function is reduced exhibit abnormal behavior characteristics.
Another semaphorin gene has been identified in several strains of poxvirus. This semaphorin is found in vaccinia virus (Copenhagen strain) and is encoded in an open reading frame (ORF) known as A39R. The A39R encoded protein has no transmembrane domain and no potential membrane linkage and is known to be a secreted protein. A variola virus ORF also contains sequences that share homology with the vaccinia virus ORF A39R at the nucleotide level and the amino acid level. Another viral semaphorin, AHV-sema, has been found in the Alcelaphine Herpesvirus (AHV).
Genes encoding mammalian (human, rat, and mouse) semaphorins have been identified, based upon their similarity to insect semaphorins. Functional studies of these semaphorins suggest that embryonic and adult neurons require a semaphorin to establish workable connections. Significantly, the fast response time of growth cone cultures to appropriate semaphorins suggests that semaphorin signaling involves a receptor-mediated signal transduction mechanism. To date, one semaphorin receptor, designated neuropilin, has been isolated using mRNA from rat spinal cord. Another receptor, designated neuropilin-2, has been suggested (Kolodkin et al. Cell 90:753–762, 1997)
Semaphorin ligands that are secreted into the extracellular milieu signal through receptor bearing cells in a local and systemic fashion. In order to further investigate the nature of cellular processes regulated by such local and systemic signaling, it would be beneficial to identify additional semaphorin receptors and ligands. Furthermore, because virus encoded semaphorins are produced by infected cells and are present in viruses that are lytic (poxviruses) and viruses that are not known to be neurotropic (AHV), it is unlikely that their primary function is to modify neurologic responses. It is more likely that the virus encoded semaphorins function to modify the immunologic response of the infected host and it is likely that mammalian homologues to virus encoded semaphorins function to modify the immunologic response. In view of the suggestion that viral semaphorins may function in the immune system as natural immunoregulators it would be beneficial to identify semaphorin receptors as therapeutic agents for enhancing or downregulating the immune response.